Audion amplifier



April 11, 1933.

E flefscfar aufpu/f 1 /73 A. w. BARBER 1,903,542

AUDION AMPLIFIER Filed Feb. 12, 1951 2 Sheets-Sheet 1 S Qace Carrel)?April 11, 1933. w BARBER AUDION AMPLIFIER Filed Feb. 12, 1931 2Sheets-Sheet 2 Patented Apr. 11, 1933 mam) w. BARBER, or aoonron, newmnsnv AUDION mmrma REISSUED dpplication filed February 12, 1931. Serial110. 515347.

This invention relates to audion amplifiers and more particularly toimproved methods of and circuit arrangements for automatlcallycontrolling the gain of the amplifier.

In the reception of modulated carrier wave signals, it is desirable tooperate the high frequency amplifier at maximum sensitiv ty so long asthe received signal energy 1s msuflicient to produce a predeterminedinput voltage across the detector, and to reduce the sensitivity of theamplifier for stronger signals at such rate that the detector inputremains substantially constant. Some types of automatic gain control,and particularly those employing a diode both as a demodulator of thereceived signal and as the rectifier for producing a direct current gaincontrol voltage, are open to the disadvantage that the sensitivity isautomatically reduced for all signal inputs above the threshold valuewhich will actuate the receiver, thus defeatin the purpose of theautomatic gain control wEich is to obtain a constant output level overthe widest possible range of received 25 signal energy.

Objects of the resent invention are to provide methods 0 and circuitarrangements for automatic gain control which are characterized by thesubstantial postponement of 9 a reduction in sensitivity until thereceived signal energy has risento the value corresponding, at maximumsensitivity, to the desired output level. Further objects are to providemethods of and circuit arrangements for opposing the effects of theautomatic gain control voltage produced during reception of signals ofrelatively low magnitude.

More particularly, objects are to provide methods of and circuitarrangements for automatically controlling the gain of an amplifier bythe use of two biasing potentials which vary automatically with receivedsignal energy; the effects of the two bias poten- 5 tials being inopposition and substantially nullifying each other for all signals of amagnitude less than that efiective, at maximum sensitivity, to producenormal output.

These and other objects of the invention will be apparent from thefollowing specification when taken with the accompanying drawings, inwhich Fi l is a curve sheet showing the relations ip between amplifierinput and output for an ideal gain control system, and the relationshipactually obtained with one embodiment of the invention,

Fig. 2 is an explanatory diagram based. upon the volta e-curr'ent"characteristic of an audion ampli er,

Fi '3 is a schematic circuit diagram of one orm of gain control systemconstructed in accordance with the invention,

Fig. 4 is a curve sheet showing the variations, with carrier wavevoltage, of the auto- (:3 matic bias potentials, and d Fig. 5 is acircuit diagram of one embodiment of the invention.

As usually understood in connection with radio receivers, an automaticvolume or au- (3 tomatic gain control would, if ideal operation obtains,maintain maximum sensitivity so long as the received signal energy wasinsufiicient to' raise the detector input voltage to a predetermined ornormal level, and 7 would reduce the sensitivity as the signal energyincreased beyond that critical value, .the decrease insensitivity forincreasing signal strength being at such rate that the detector inputremained substantially constant.

In Fig. 1, the curve A represents the relationship which would existbetween the amplifier input and output voltage if an ideal automaticgain control were employed. As the incoming carrier voltage E increases8 from its threshold value, the amplifier gain should remain constant atits maximum value until the incoming signal reaches that value e whichbrings the detector input voltage E to the predetermined value -E atwhich the receiver is" to be operated. In other words, the amplifieroutput should be proportional to the input for all signals below thenormal signal input voltage which, at maximum sensitivity, correspondsto normal ampli--' fier output or detector input voltage. For all highersignal voltages, the amplifier output should remain constant. I

Curve B shows the relationship existing between amplifier input E andamplifier out- 1 put E in one known form of automatic gain controlsystem in which a diode serves both as the signal demodulator and thesource of automatic gain control potential. A comparison of curves A andB demonstrates that such systems do not satisfy the essentialrequirements of a. gain control which, over a wide range of impressedsignal voltages, will maintain constant output, since the output doesnot reach its intended or normal value E until the signal voltage risesto avery high value.

Systems of this old type limit the output to values much less thanproportional to input for all signals, and their operation is notcharacterized by a quick rise to normal output, followed by constantoutput, as the signal voltage is increased.

In accordance with the present invention, the sensitivity of theamplifier may be kept approximately constant until the carrier inputreaches its normal value e and the normal'ouput voltage E is reached atsomewhat greater carrier input. The curve C of Fig. 1 illustrates thecontrol obtained with one embodiment of the invention, and it, will benoted that this curve is a close approximation to the ideal operatingcharacteristic A.

To defer the reduction in sensitivity, use is made of two biaspotentials which vary automatically with signal strength and whic forany given change in signal strength tend to change the amplifier gain inthe opposite sense.

One bias potential is obtained, in the known manner, by rectification ofthe carrier and this direct current voltage increases as a substantiallylinear function of the carrier when, as is the usual case, a linearrectifier is employed.- The other component of the control potentialcomprises the direct current drop across a. resistor in that part of thecathode circuit which is common to the grid and plate circuits of thecontrolled amplifier, i. e'., an autoor cathode bias. Although thecathode bias potential doesnot vary as a direct result ofchangingcarrier wave energy, its magnitude is dependent, at least in part, uponthe magnitude of the rectifier bias potential which varies automaticallywith signal strength. i v

- The relative effects of these two bias potentials will be apparentfrom a consideration of the amplifier voltage-current characteristic,Fig. 2, and the circuit diagram, Fig. 3. The curve D of Fig. 2 shows therelationship between grid bias E and space current I for an amplifiertube and, as shown in Fig. 3, the effective grid bias, potential E isdetermined by the algebraic sum of three components, two of which varyautomatically with signal strength.

As shown diagrammatically in Fig. 3, the input circuit 1 of theamplifier tube 2 is connected to the tube grid and, through a.

blocking condenser 3, to ground. The resistor 4 is connected betweenthetube cathode and ground, and therefore the direct current potentialestablished across this resistor by the space current flow provides abias potential efi'ective between the amplifier grid and cathode. Theplate circuit of tube 2 is connected by a network 5, (which may besimply an interstage cou ling or may include additional amplifiers; tothe plate of the diode rectifier 6. The circuit between the diode plateand cathode includes the radio frequency circuit 7 and the resistance 8which is by-passed for radio frequency currents by a condenser 9. A lead10 connects the low potential terminal of the amplifier input circuit 1to that end of resistance 8 which is spaced from the diode cathode, andthe latter is connected to ground through a battery 11. No filternetwork is shown for eliminating audio or radio frequency componentsfrom the direct current bias potential feed back to amplifier 1 sincethe exact form of such network has no bearing upon the theory of op-.eration of the gain control system.

In the absence of any signal voltage E across the amplifier input, thebias E between the amplifier grid is:

where E, is the potential drop due to the flow of space current throughthe cathode bias resistor 4, E is the potential drop arising from theflow of space current in the diode, and E, is the voltage across thatportion of battery 11 which is between the diode cathode and ground.

Referring now to Fig. 2, the point 0 on curve -D is the operating pointfor maximum sensitivity of the amplifier tube, and operation about thispoint of the characteristic is effected by the application of a negativebias voltage e As is well known, the slope of a line b drawn throughpoint 0 and the origin 0 gives the magnitude of the resistance 4 whichwould, for space current flow corresponding to point 0, give a cathodebias potential of e In accordance with the invention, the cathode biasresistance 4 is substantially larger than the magnitude indicated by theslope of line b, the slope of line 0 being so chosen, with respect tothecurvature of the characteristic D, that the cathode bias voltage Erepresented graphically by the projection of line a on the voltage axis,decreases rapidly as the bias potential is increased negatively to shiftline 0 towards the left.

As indicated in Fig. 2, the initial bias e is the algebraic sum of thenegative cathode bias E,, the negative rectifier bias E which isestablished across rectifier circuit resistance 8 by the. space currentfor zero input voltage E and the positive bias potential E, from thesource 11.

As shown graphically in Fig. ,2,-the highcathode bias resistor 4 effectsa substantial reduction in the net change in'bias potential a, when, forsmall signal inputs less than the normal input, the rectifier bias Eincreases with increasing signal stren When the rectifier bias E, isnumerica ly equal to the fixed bias E the operating point 0' is locatedby drawing through origin 0, a line 0' parallel to the line 0. It ,willbe noted that the cathode bias potential has dropped materially andtherefore the net change in the gain control bias e is substantiallyless than the change in the rectifier bias E The relationship betweenthe cathode bias potential E the rectifier bias potential E and the gaincontrol bias E for increasing signal strength E, is illustrated in Fig.4. For the particular gain control system from which the data wasobtained, the decrease in cathode bias was somewhat less than theincrease in rectifier bias for values of E less than normal input e thusaccounting for the fact that the control curve C, Fig. 1, is slightlylower than the inclined branch of the ideal control curve A. Forsomewhat greater values of signal input E, but little change in thecathode bias otential E is produced by increasing recti er potentials Eand therefore the curve e is substantially parallel to the rectifierpotential curve E One practical embodiment of the invention, asillustrated in Fig. 5, comprises a twostage tetrode amplifier workinginto a diode rectifier. The input circuit of each tetrode amplifier 12includes in series, the inductance 13, tuning condenser 14 and ablocking condenser 15 of 0.1 microfarad capacity.

A cathode resistance 16, by-passed for radio frequency currents, isincluded in each amplifier circuit. The circuit of diode 17 comprisesthe tuned radio frequency impedance 18 in series with a resistance 19 ofapproximately 1 megohm which is by-passed for radio frequency bycondenser 20. The audio frequency potential developed across resistance19 may be passed to an audio frequency amplifier, not shown, and therectified direct current potential is passed, by lead 21 to the gridcircuits of the amplifiers, resistances 22 of a half megohm beingincluded to suppress audio frequency components from the bias potentialspassed to the amplifier grids. The positive bias potential necessary toreduce the high negative bias components, due to the noload spacecurrent fiow in the respective tubes, to the appropriate value formaximum sensitivity is supplied by a battery 23. The values stated areappropriate when the'tetrodes are of the commercial 224 type and thediode is of the commercial 227 type, with grid and plate connectedtogether.

It will be apparent that the use of two bias potentials which varyautomatically with signal strength affords wide latitude in the designand construction of automatic ain control systems. While I have describeone par ticular type ofcontrol, it will be apparent that the effects ofchanging cathode blas p0- tentiak and changing recti r potential may, ifdesired, be combined to produce control characteristics substantiallydifferent from that indicated by curve C of Fig. 1.

It is also apparent that the invention is not limited to systems inwhich the rectifier has the further function of demodulating the signalstransmitted by the amplifier, or in which the rectifier input voltage isderived from the amplifier output.

I claim:

1. In the operation of an electrical wave amplifier working into arectifier and subject to signals of varying magnitude, the method ofautomatically controlling the gain of said amplifier, which comprisesimpressingupon the amplifier, input circuittwo discretedirect currentpotentials which vary automatically and according to different functionsof the magnitude of the incoming electrical wave, one of said potentialsbeing predominately effective for weak signals and the otherpredominately effective for strong signals.

2. In automatic Volume control apparatus wherein two discrete potentialsare employed to control amplifier gain, means whereby one of saiddiscrete potentials is obtained by rectification of the amplifieroutput, and means whereby the second potential is derived from the spacecurrent flow in the cathode circuit of the amplifier.

3. The method of operation of an electrical wave amplifier so as tosecure automatic gain control thereof, which comprises applying to saidamplifier a gain control potential which varies automatically with themagnitude of the input of said amplifier and applying to said amplifiera second gain control potential which varies automatically as a functionof said first gain control potential.

4. The method of operation of an electrical wave amplifier so as tosecure automatic gain control thereof, which comprises applying to saidamplifier a gain control potential which varies automatically with themagnitude of the input of said amplifier and applying to said amplifiera second gain control potential which Varies automatically as a functionof said first gain control potential, said second gain control potentialbeing obtained by the voltage drop due to the flow of current in thecathode circuit of said amplifier.

5. The method of operation of an electrical wave amplifier so as tosecure automatic gain control thereof, which comprises applying to saidamplifier a gain control potential which varies automatically with themagnitude of a the input of said amplifier and applying to varyingmagnitudes so as to secure an automatic control of the gain of saidamplifier, which comprises impressing a direct current potential varyingwith signal strength upon the input circuit thereof and impressing uponsaid input circuit a second direct current potential automaticallydecreasing in magnitude as the strength of the received electric signalsincreases, both of said direct current potentials functioning as gaincontrol voltages.

7 In an electrical wave transmission system, the combination with anelectron tube amplifier having input and output circuits, and means forautomatically impressing u on said amplifier a gain control potentialwhich varies in magnitude with changes in the magnitude of theelectrical waves to be amplified, of means substantially preventing saidautomatic gain control potential from affecting the amplifier gain untilthe electrical waves increase in magnitude to a predetermined value,said last mentioned means including a cathode bias resistancemetallically connected in the input circuit of said amplifier.

8. In an electrical wave transmission system, the combination with anelectron tube amplifier, a rectifier, and means impressing upon saidamplifier a gain control voltage derived from said rectifier byrectification of alternating currents traversing said s stem, of meansimpressing upon said ampli er a second gain-control potential of a manitude determined as a function of said 'rst control voltage andoperative conjointly therewith so as to prevent said first controlvoltage from being substantially effective un-- til it reaches apredetermined value.

9. In an electrical wave amplifier, the com- 7 bination with a vacuumtube and input and output circuits therefor, a rectifier and meansimpressing 'upon the input circuit thereof electrical waves varying inmagnitude with v the electrical wave input to said vacuum tube, andcircuit elements associated with the rectifier for impressing upon saidtube a gain control potential varying with rectifier out put, of meansin circuit with said tube for developing a second gain control potentialwhich varies in magnitude with the gain control potential impressedthereon by said rectifier and operates conjointly therewith so as toprevent said first controlled voltage from being substantially effectiveuntil i it reaches a predetermined value.

10. An electrical wave transmission system of the type including an amlifier and a rectifier for'impressing upon said amplifier a control gridbias potential varying automatically with changes in the amplitude ofthe electrical waves impressed upon said amplifier, characterized by thefact that the oathode circuit of said amplifier includes a bias resistoracross which the space current flow establishes a bias potential, saidresistor having a magnitude such that for a predetermined range ofamplifier input voltages the changes in, cathode bias potentialsubstantiall ofl'set changes in the rectifier bias potentlal.

11. In a carrier wave receiver, the combination with a carrier waveamplifier, a diode rectifier having input and output im edancesassociated therewith, means coup ing the amplifier output circuit to theinput impedance of said. diode rectifier, and circuit elements fortransferring back to said amplifie'r a gain control voltage developedacross the output impedance of said diode rectifier, of a cathode biasresistor in said amplifier of such magnitude as substantially topostpone reduction in amplifier gain until said gain control voltagereaches a predetermined value.

- 12. In a carrier wave amplifier, the combination with an amplifiertube having input and output circuits, a diode rectifier having an inputcircuit across which is developed av carrier voltage varying withreceived carr1er wave voltages, of means operative in the absence ofrecelved carrier waves to has said amplifier tube to maximumsensitivity, said

